The voluntary efforts to curb greenhouse gases -- if respected -- would only yield a third of the cuts needed by 2030 to keep Earth from overheating, according to a UN Environment Programme report.
Countries have made "an historic level of commitment" with their pledges, UNEP Executive Director Achim Steiner said in a statement.
"However, they are not sufficient to limit global temperature rise to the recommended level of two degrees Celsius (3.6 degrees Fahrenheit) this century."
The emissions reduction plans -- called Intended Nationally Determined Contributions, or INDCs -- would result in a temperature hike of 3.0 C (4.8 F) or more by 2100, UNEP said.
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Other scientific estimates have varied between 2.7 C and 3.5 C.
The annual "Emissions Gap" analysis tracks the difference between projected CO2 pollution, on the one hand, and the levels required to stay under the UN 2 C target, on the other.
This year's report is the first to take into account greenhouse gas reduction promises made ahead of the November 30-December 11 summit, tasked with delivering the first-ever universal climate pact.
The INDCs would shave 6 billion tonnes off the top, leaving the total at 54 billion tonnes.
But to keep the 2 C target on track, total emissions in 2030 should not exceed 42 billion tonnes, according to the UN's climate science panel.
That means that the INDCs only cover a third of the 18 billion tonne gap between the current trajectory, and where we need to be in 2030.
Moreover, it notes, greenhouse gas output would still be rising in 2030.